Read the article, The Basics of Data Classification available from Cartography Guide by Axis Maps, licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License. to have an idea on how to classify geospatial data for thematic methods.
Moreover, ESRI Resources Page has a very informative article on Data classification methods and the ArcGIS Blog entry Better Breaks Define Your Thematic Map’s Purpose by Jim Herries gives you the opportunity to experiment with a thematic map with actual data so that you can see how the map changes based on the classication method.
If you’re further intrigued by the topic, the article Visualizing Cancer Data using GIS by Cinthia Brewer on how classification impacts mapping in the context of public health.
For Greek educators and students, the chapter Data classification in the book by Kokla, M., & Nakos, B. (2024). Thematic Cartography [Undergraduate textbook]. Kallipos, Open Academic Editions, provides information on the subject in Greek.